Callback support for Lite

[lightningforever]

What does this PR do?

Arbitrary callbacks are now supported in Fabric. The user is responsible for invoking them, and fabric has a convenience method to trigger all callbacks at once for a given "event".

fabric = Fabric(callbacks=[callback0, callback1])
fabric.call("on_train_end")
fabric.call("on_batch_start", batch, batch_idx)  # arguments can be passed down as well

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[lantiga]

Great, just a naming proposal